This is the first application filed for the present invention.
Not Applicable.
The invention relates in general to fluid filtration systems and, in particular, to a power supply for extending the service life of an electrostatic air filtration system.
Electrostatic air filtration systems can be generally categorized into one of two types: precipitators, and charged-media type air filters. In precipitators, corona wires ionize air contaminants, and the ionized contaminants precipitate onto oppositely charged collector plates. High voltages are used to ionize the airborne contaminants in order to enhance precipitation of the contaminants. The precipitation of ionized contaminants occurs due to Coulomb forces induced by electrostatic fields surrounding the corona wires, which produce voltage gradients. Air filtration efficiency is increased by steep voltage gradients. Steep voltage gradients can be achieved by increasing a voltage applied between the corona wires and the collector plates of an air filtration unit. Ionization of air contaminants is effected largely by dissociating electrons from air contaminating molecules or air contaminating particles. Once dissociated, the ionized contaminants travel towards the collector plates and the dissociated electrons travel towards the corona wires.
There is a degree of ease with which air contaminants ionize. As air itself can be ionized, precipitator air filtration systems are adapted to precipitate air contaminants which ionize easier than air. The ionization is also affected by conditions such as ambient temperature, relative humidity, air pressure, etc. At very steep voltage gradients, air itself ionizes. As opposed to contaminant ionization, there is a plentiful supply of air available. Therefore, on ionizing air, a continuous path of ions travelling towards the corona wires is formed. The ionized path is localized and provides an electrical discharge path between the high voltage corona wires and the grounded collector plates, temporarily destroying the electrostatic field therebetween. The contaminant ionization efficiency is reduced, therefore reducing the filtration action of the apparatus. The ionized discharge path can be observed as a spark and therefore referred to in the art as a xe2x80x9cspark-overxe2x80x9d.
High voltage spark-overs provide a path for large load currents to flow between the high voltage corona wires and the grounded collector plates, and is equivalent to a short circuit. Typically the high voltage electronics of the high voltage power supply are designed to apply high voltages to the high voltage corona wires using load currents that can sustain only low rates of discharge. Operating the high voltage electronics at high load currents can produce excessive heating and therefore component wear at an accelerated rate. Spark-overs also destroy metal surface treatments used in manufacturing the electrostatic filter electrodes, reducing their ability to withstand high humidity, microbial damage, and other corrosives.
Several power supplies have been invented for precipitator-type electrostatic air filtration systems.
For example, U.S. Pat. No. 4,936,876 entitled xe2x80x9cMETHOD AND APPARATUS FOR DETECTING BACK CORONA IN AN ELECTROSTATIC FILTER WITH ORDINARY OR INTERMITTENT DC-VOLTAGE SUPPLYxe2x80x9d, which issued Jun. 26, 1990 to Reyes describes an electrostatic precipitator for cleansing flue gases from industrial plants. The apparatus is designed to detect back corona discharges in a dust layer precipitated on contaminant collecting electrodes. The apparatus makes periodic upward adjustment of the output voltage until spark-overs occur. Based on the setting at which the spark-over occurs the apparatus is adjusted to provide optimal contaminant precipitation for a period of time.
U.S. Pat. No. 5,471,377, entitled xe2x80x9cPROCESS FOR CONTROLLING A POWER SUPPLY WHICH SUPPLIES POWER TO AN ELECTROSTATIC FILTER IN WHICH SECONDARY CIRCUIT STATES ARE DETERMINED BASED ON MEASURED PRIMARY CIRCUIT VALUES AND IN WHICH SHORT CIRCUITS ARE DETECTEDxe2x80x9d, which issued Nov. 28, 1995 to Donig et al., describes a method of monitoring a voltage on a primary coil of a high-voltage transformer for voltage drops signifying short circuits at the output. The apparatus counts short circuit occurrences over time and shuts off the power supply on detecting an excessive number of short circuit occurrences.
U.S. Pat. No. 5,920,474 entitled xe2x80x9cPOWER SUPPLY FOR ELECTROSTATIC DEVICESxe2x80x9d issued Jul. 6, 1999 to Johnson et al., describes a method of providing a substantially ripple-free DC power for improved operation of an electrostatic filter.
Three-phase AC power is amplified and rectified to provide the DC output. The input voltage is assumed to be stable, and there is no provision for controlling spark-overs due to over-voltages in the input power.
U.S. Pat. No. 4,779,182 entitled xe2x80x9cPOWER SUPPLY FOR AN ELECTROSTATIC FILTERxe2x80x9d issued Oct. 18, 1988 to Mickal et al., describes a power supply adapted to decouple an output stage from an input stage on detecting short circuits by monitoring a feedback signal representative of the output voltage. The stress put on the power supply circuitry by high current flows generated by short circuits is therefore reduced.
Electrostatic air filters of the charged-media type remove contaminants form air using polarized dielectric fibrous filter pads sandwiched between a highly charged positive electrode and a grounded electrode. The electrodes are typically wire mesh or screen, though other materials are sometimes used. A strong electric field created between the positive and grounded electrodes polarizes the dielectric filter medium, so that positive and negative poles appear on the surface of each fiber. The electric poles attract particulate matter from air passed through the filter. The efficiency of the filter is thus dramatically enhanced.
In general, charged-media type air filters are used in less harsh environments than precipitators. The demands on the power supply are therefore somewhat less demanding but the power supplies for charged-media type air filters are subject to the same stresses as those for precipitator-type filters. Power supplies for charged-media type air filters are typically similar to a power supply described in Canadian Patent No. 1,272,453 which issued to Joannou on Aug. 7, 1990. That power supply includes an electronic oscillator that drives a transformer. The transformer outputs about one tenth of the desired output voltage. A voltage multiplier section boosts the output voltage to the 5-10 Kv. range required to drive the filter. The three sections are mounted on a printed circuit board.
The existing power supplies for electrostatic air filters of the charged-media type have many disadvantages. They do not isolate the input power from the output, so the input power must be isolated or grounded, which is difficult to ensure. Furthermore, they typically step up the output voltage in direct proportion to the input power. Consequently, if the input power is unstable or the input voltage is higher than specification, the output voltage may be too high which can cause arching or shorting in the filter. A further problem is associated with the construction of the power supply, which makes it vulnerable to corrosion, dust contamination and even tampering.
There therefore exists a need for a more reliable power supply for electrostatic air filtration systems. In particular, there exists a need for a power supply for an electrostatic air filter of the charged-media type that overcomes the above-noted problems of power isolation, output regulation and power supply construction.
It is an object of the invention to extend the service life of electrostatic air filtration systems by providing a more reliable power supply.
In accordance with one aspect of the invention, a power supply for an electrostatic air filtration unit is provided. The power supply includes an input stage, an isolation stage and an output stage. The input stage includes an oscillator driven by input power and provides an oscillator output voltage. The isolation stage electrically isolates the output stage from the input stage. The output stage includes an output amplifier driven through the isolation stage by the oscillator. The output amplifier provides an output voltage on an output electrode. The output stage further includes a voltage regulator for preventing the output voltage from exceeding a predetermined maximum voltage. By limiting the output voltage to a maximum voltage, the output of the power supply is rendered insensitive to over-voltage fluctuations in the input power, thereby inhibiting arching.
In accordance with another aspect of the invention, the power supply is encased in a resin to extend the operational life of the power supply by providing resistance to tampering, and dramatically improving resistance to environmental damage caused by water infiltration, corrosion and microbial attack.
The advantages include a dramatically improved resistance to high humidity and corrosion. The electrical isolation of the output stage from the input stage permits the power supply to be connected to an ungrounded power source. A clamped high voltage output provides spark-over suppression, and tamper-proof, waterproof encasement extends the service life of the air filtration apparatus.